(1) Field of the Invention
The present invention relates to a catalyst system for manufacturing polyethylene terephthalate (PET) and a process of producing PET using the catalyst system. In particular, the present invention relates to a novel catalyst system unique in producing PET from terephthalic acid and ethylene glycol. The unique catalyst system always includes antimony and generally includes cobalt and/or manganese and/or zinc. The catalyst system of the present invention may also include magnesium and calcium. In the manufacturing process, using the catalyst system of the present invention, the polymerization rate is drastically increased without substantially sacrificing color and clarity of the polymer produced.
(2) Prior Art
Polyethylene terephthalate is prepared from one of two processes, namely: (1) the DMT process and (2) the TA process. In the DMT process dimethyl terephthalate (DMT) is reacted with ethylene glycol in an ester interchange reaction (trans-esterification) to yield bis(2-hydroxyethyl) terephthalate (monomer) and methanol. Because the reaction is reversible, it is necessary to remove the methanol to completely convert the raw materials into monomer. It is known to use manganese and/or cobalt and/or zinc, etc. in the ester interchange reaction. The catalyst activity is then sequestered (arrested) by introducing phosphorus at the end of the ester interchange reaction. The monomer then undergoes a condensation reaction (polycondensation) which polymerizes the monomer to PET. When the monomer undergoes polycondensation, the catalyst most frequently employed is antimony. If the catalysts employed in the ester interchange reaction are not arrested with phosphorus, the resultant polymer easily degrades (thermal degradation) and has a very unacceptable yellow color. See Kinetic Aspects of Catalyzed Reactions in the Formation of Poly(ethylene terephthalate) by S. G. Hovenkamp, published in the Journal of Polymer Science, Part A-1, Vol. 9, pages (3617-3625) 1971.
The following patents are exemplary of the DMT process in which manganese and cobalt are employed as catalysts in the ester interchange step in producing PET. U.S. Pat. No. 3,907,754 to Tershansy et al., U.S. Pat. No. 4,010,145 to Russin et al., and U.S. Pat. No. 4,122,107 to Kenney disclose using manganese, cobalt, zinc, or other metals, or their salts in the ester interchange or transesterification process. Additionally, Reissue U.S. Pat. No. 32,765 to Callander et al. also discusses using a cobalt compound in making PET. Each of the patents also discloses employing phosphorus at the end of the transesterification process to sequester or arrest all catalytic activity.
The second method for making PET is to react terephthalic acid (TA) with ethylene glycol in a direct esterification reaction producing bis(2-hydroxyethyl) terephthalate or "monomer" and water. This reaction is also reversible and thus can be carried to completion by removing the water during the reaction. The direct esterification step does not require a catalyst and conventionally no catalysts are employed. Just as in the DMT process, the monomer then undergoes polycondensation to form PET. The polycondensation reaction typically uses antimony as a catalyst.
In the direct esterification step in the TA process, no catalysts are employed. In fact, when materials such as manganese, cobalt, or zinc employed in the ester interchange in the DMT process are tried in the direct esterification in the TA process, such materials have little or no catalytic effect.
In summary, if the DMT process is employed to make PET, there are two steps, namely: ester interchange (transesterification) and polycondensation. All catalytic activity is arrested at the end of the first step by introducing phosphorus. If the TA process is employed to make PET, there are also two steps, namely: direct esterification and polycondensation. Generally no catalyst system is employed in the direct esterification step. In both processes, antimony is the preferred catalyst in the polycondensation step.
It is the primary aim of the present invention to increase the polymerization rate in the TA process for making polyethylene terephthalate.
Another primary object of the present invention is to create a catalyst system which substantially increases the polymerization rate for producing PET from TA, whereby the catalyst system can be added at anytime before the beginning of or during the polycondensation step.